Flame sensors are used to sense the presence or absence of a flame in a heater or burner, for example, or other apparatus. Flame detector systems are available to sense various attributes of a fire and to warn individuals when a fire is detected. For example, flame detector systems utilizing ultraviolet (“UV”) sensors are known. In the flame detector system, UV radiation emitted from the flames of a fire is detected by the detector's UV sensor. When a sufficient amount of UV radiation is detected, the flame detector system goes into alarm to warn individuals of the flame.
Typically, the UV sensor can be constructed of a sealed UV glass tube with a pair of electrodes and a reactive gas enclosed therein. A constant voltage is typically applied across the UV sensor in order to adequately sense UV radiation. In the presence of UV radiation of a certain wavelength (typically in the range of 100-300 nm), the sensor discharges the voltage to indicate detection of UV radiation. After the UV sensor discharges, the voltage across the sensor must be refreshed to allow the sensor to continue to detect UV radiation. Typically, once a UV sensor discharges, it is refreshed at a periodic interval.
The performance of the UV sensor is known to degrade over time. It can therefore be important to monitor the performance or “health” of the UV sensor to identify when performance of the sensor degrades. One mode of failure is the state where the current flow across the two electrodes occurs spontaneously without the presence of the ultraviolet light from the flame. In this case the sensing tube is indicating the presence of a flame when in fact no flame is present. This condition is commonly referred to in the industry as “run-on”. A drawback for flame detector tubes that use photoemission for a metal surface followed by a discharge is that the when the tubes degrade they can fail do to run-on. Run-on is the condition in which the tube keeps firing even after ultraviolet light is not present.
In an effort to address the foregoing difficulties, it is believed that additional electrodes that are sensitive to a breakdown condition can be utilized to detect run-on conditions.